(1 ) Field of the Invention:
The present invention relates to a digital magnetic recording and reproducing apparatus which records and reproduces a digital code signal, particularly to a digital magnetic recording and reproducing apparatus provided for recording a digital code signal which is previously encoded with a first code conversion and for restoring the original digital code signal by decoding the recorded signal using a second code conversion which is complementary to the first.
(2) Description of the Prior Art:
Recently, in accordance with the rapid progress of digital techniques, various types of the digital magnetic recording and reproducing apparatus provided for storing various kinds of coded information signals including, for example, coded picture signals have been investigated. Particularly, for the purpose of preventing the occurrence of erroneous codes caused by the inevitable deterioration of frequency response and signal-to-noise ratio of coded signals reproduced in those known apparatus, various kinds of improving methods have been investigated. Typically these improving methods employ the use of conventional methods applied in ordinary digital transmission systems in the magnetic recording and reproducing of digital signals. For example, the so-called non-return-to-zero-inverse system wherein a pseudo-random pulse sequence is modulo-2-added to the digital signals to be transmitted, the so-called partial response system wherein the code of digital signals to be transmitted is converted partially by utilizing the deformation of waveform which is caused by the performance of the transmission system, and the dc level restoration technique which is effected by the level comparison between the received digital signals and the threshold level generated by detecting peak levels thereof have been used.
However, those conventional improving methods, which present remarkable effects for the ordinary transmission systems, cannot present sufficient effects for recording and reproducing magnetically digital signals because of the problems which are peculiar to the magnetic recording and reproducing apparatus.
The above-mentioned problems are as follows.
(1) In magnetic recording and reproducing of digital code signals, when those digital code signals to be recorded and reproduced contain the portions in which almost the same signal level continues at a comparatively long interval, it is impossible to reproduce the clock pulse sequence continuously which is required for correcting signal levels and decoding of reproduced digital code signals and which is extracted from the reproduced digital signals by means of detecting polarity inversions, so that it is impossible also to properly restore the recorded digital code signal. Accordingly, a code conversion is desired which prevents the continuation of the same signal level in the digital code signal to be recorded.
However, the deformation of the reproduced digital code signal, which is caused by the response of magnetic heads, is so excessive that the above problem is not prevented by the above-mentioned known improving methods.
(2) The higher the density of the magnetic recording, the more excessive the variation in reproduced signal levels. Consequently, even if the aforesaid partial response system is adopted for magnetic recording and reproducing of the digital code signal, the correct discrimination of levels of the reproduced digital signal is disturbed by such excessive variations.
(3) As is true of most magnetic recording and reproducing apparatus, rapid variations in the reproduced signals caused by partial variations in the response of the magnetic carriers, for example, magnetic tapes, cannot be avoided. Therefore, in order for the level comparison to correct the reproduced signal level the threshold level used for the level comparison must follow the above rapid variations in the response faithfully. However, sufficient faithfulness cannot be obtained by the above-mentioned conventional dc level restoration effected by the known simple peak detection.
(4) The deterioration of response in the lower frequency range, which is caused by rotary transformers and magnetic heads, is inevitable in the magnetic recording and reproducing apparatus. Accordingly, when the digital code signals to be recorded and reproduced contain portions in which almost the same signal level continues for a comparatively long interval, as mentioned above, excessive shifts of levels of the reproduced signal are caused by the aforesaid deterioration of response in the far lower frequency range, and, as a result thereof, the erroneous codes frequently occur.
Thus, as explained above, the digital magnetic recording and reproducing apparatus has many difficult problems which are peculiar thereto which do not occur in the ordinary digital transmission systems.